1. Field of the Invention
The present invention relates to improvements of a multistage transmission capable of carrying out seven or more forward gear stages.
2. Discussion of the Related Art
A multistage transmission in which a plurality of planetary gear sets, clutches and brakes are employed has been widely used as an automatic transmission for a vehicle. And, a multistage transmission capable of providing multistage forward transmission of seven or more stages has been proposed as such an automatic transmission. An automatic transmission described in Patent Documents 1-8 are examples thereof, which are capable of providing multistage forward transmission of 9 stages through 12 stages by employing three or four planetary gear sets.
[Patent Document 1]
    Japanese Patent Publication No. 2002-206601[Patent Document 2]    Japanese Patent Publication No. H8-105496[Patent Document 3]    Japanese Patent Publication No. 2000-199549[Patent Document 4]    Japanese Patent Publication No. 2000-266138[Patent Document 5]    Japanese Patent Publication No. 2001-82555[Patent Document 6]    Japanese Patent Publication No. 2002-227940[Patent Document 7]    Japanese Patent Publication No. 2002-295609[Patent Document 8]    Japanese Patent No. 2956173
In such a multistage transmission, although it is preferable that (a) the overall range of a transmission ratio is sufficiently wide, (b) steps of respective transmission ratios are arranged to be as equal as possible, (c) the number of engagement elements (such as clutches and brakes) is as small as possible, (d) changes in engagement of respective engagement elements in shifting actions are easy, (e) the number of planetary gear sets is as small as possible, and (f) a gear ratio ρ (number of teeth of sun gear/number of teeth of ring gear) of planetary gear sets is, for example, in a range of 0.3 through 0.6 in view of making the transmission compact, it is difficult to meet all these requirements, wherein the requirements are not necessarily sufficiently satisfied. For example, in a multistage transmission described in Patent Document 1, it is necessary to change a gripping engagement of four engagement elements at most when shifting a gear stage, wherein complicated and accurate shifting action control of a transmission is required, and at the same time there is a possibility for a shock to occur in speed change. Further, there is a problem in that, since the gear ratio ρ of planetary gear sets is 0.273 through 0.778, it is difficult to compactly construct the transmission.